1. Field of the Invention
The present invention relates, in general, to electrically enhanced air filtration and, more specifically, systems and methods for increasing efficiency of electrically enhanced air filtration while avoiding arcing and minimizing the loss of collection efficiency which results from charge accumulation on the fibers of the mechanical filter utilized.
2. Relevant Background
Gas filtration, and more specifically air filtration, is used in a wide variety of applications ranging from automobiles, homes, office buildings and manufacturing facilities. In many cases filtration systems are used to remove pollutants such as dust, particulates, microorganisms and toxins from breathing air, although filtration systems and processes may be used to purify manufacturing environments, process gasses, combustion gasses and the like.
One particular application is for heating, ventilation, and air conditioning (HVAC) systems within buildings. HVAC systems comprise a motor and blower that moves air from a supply through ductwork that distributes the air throughout building spaces. The air supply may be outside air, re-circulated air from inside the building, or a mixture of outside and re-circulated air. Conditioning systems such as heat exchangers, humidifiers, dehumidifiers, and the like are positioned in-line with the ductwork to adjust various characteristics of the supplied air before it is delivered to building spaces. Air filtration systems are placed in-line with the ductwork to filter out particulates and organisms from the air that are present within the flow of air.
Mechanical filters consist of a flat, or pleated, mat of fibers contained in a supporting frame. The filter is sufficiently porous to allow air flow through the filter. In operation, mechanical filters capture particulates and organisms on the filter fibers as the air stream passes through the filter. In order to capture smaller particles, the density of fibers is increased to reduce the space between individual fibers. The smaller the space between the individual fibers, the smaller the size of particle that can be trapped. Unfortunately, as the openings get smaller the resistance to airflow also increases and so the energy required to move air through the filter increases significantly when higher density filters are used. Moreover, as the filter becomes loaded with captured particulates, air flow is further restricted. As a result, high efficiency mechanical filtration is not practical for many applications. Further, mechanical filters become breeding grounds for bacteria and other organisms that are captured. As a result, the mechanical filter can actually become a source of contamination.
Another type of filtration mechanism uses frictional electrostatic technology to improve particulate capture efficiency with less air restriction. Frictional electrostatic filtration uses the fact that the friction of air moving over certain types of materials causes charge transport (i.e., static electricity”) that imparts a surface charge on the filter fibers. This surface charge encourages particles that have an opposite charge to attach to the filter fiber. Because the surface charging results from the friction of air flow, electrostatic filters are “self-charging” in that they do not require externally applied electricity. In this manner, particle capture efficiency is increased without increasing the fiber density. While frictional electrostatic filtration is an improvement over pure mechanical filtration, the charge transfer caused by air movement over the filter is relatively modest. Also, the particle efficiency is only improved for particles that have an opposite charge to the filter media. For electrically neutral particles the filter capture efficiency is similar to mechanical filters. Additionally, as particulate matter collects on the filter's fibers they reduce the frictional effect by preventing the airflow from coming into contact with the fiber's surface.
Electret filter media has been developed to enhance the capture efficiency of the filter media using built-in electric fields. When the fibers of an electret media filter are formed, the fibers are charged or polarized by application of an electric field or other technique. This charge increases the initial capture efficiency of the filter in much the same way as frictional electrostatic filters. However, as oppositely charged particles accumulate in the electret filter media the built in charge is neutralized by the particle charge, and filter efficiency returns to what would be more typical of a purely mechanical filter.
Active electrically enhanced air filtration operates on principles similar to frictional electrostatic filters, but uses externally applied electricity to polarize the filter media rather than the self-charging electrostatic effect. Using externally applied electricity enables higher voltages and corresponding higher collection efficiencies. The high voltages required large separation between some components to avoid arcing, which made early units too bulky for some applications. Also, early electrically enhanced filters were criticized because arcing problems that reduced efficiency and produced ozone and they had limited ability to remove all sizes of particulates from the air. However several improved designs have been introduced in recent years. For example, U.S. Pat. No. 5,549,735 and U.S. Pat. No. 5,593,476, which are assigned to StrionAir, Inc., which is the assignee of the present invention, describe an electrically enhanced fibrous air filter that uses polarized filter medium in combination with an upstream pre-charge system to impart a charge on particulates before they reach the polarized filter media. This system uses electrode arrangements that control arcing while at the same time producing a high polarizing field across the filter media.
In order to polarize the filter media utilized in electrically enhanced air filters, the media must be substantially non-conductive. However, the non-conductive media tends to accumulate fiber charge during operation which causes a reduction in particle removal efficiency. Over time, as charge from collected particles accumulates on the oppositely charged fiber sites this charge buildup prevents other incoming charged particles from being attracted to these collection sites. In fact, this accumulated charge will repel incoming particles away from the fibers. Additionally, in electrically enhanced air filters that utilize negative ionization to pre charge particles any pathogens trapped on the filter are bombarded by electrons and negatively charged particles which eventually results in the rupturing of the organism's cell wall killing the pathogen. It is believed that fiber charge buildup repels electrons away from the organism so it now doesn't receive the dosage needed to kill it.
Another electrically enhanced air filter system described in U.S. Pat. No. 4,940,470 and U.S. Pat. No. 5,403,383 issued to Jaisinghani et al. These designs propose a construction in which a ground electrode is in proximity or contact with the filter media while a high-voltage polarizing electrode is placed upstream of the filter. In these designs the ground electrode participates in the application of an electric field that polarizes the filter media. In some embodiments the ground electrode is in physical contact with the filter media. However, these patents and patent applications fail to teach that the ground electrode be configured to conduct accumulated charge away from the filter media. Because the ground electrode was used for field shaping, it was believed to be important that the entire downstream surface of the filter media be substantially conductive so that all of the filter surface was at a similar potential. However, it has been found that this configuration encourages arcing in pleated filter designs because the distance between the ground and the upstream ionizing electrodes varies across the pleats. Further, the continuous contact between the filter surface and the ground electrode interfered with airflow.
Published U.S. patent application 20020152890A1 to Leiser builds on the Jaisinghani et al. by suggesting that a conductive coating be applied to only a portion of the downstream side of the filter media to lessen the occurrence of arcing. While recognizing the arcing problem, the Leiser publication continues to rely on the ground electrode solely for the purpose of applying an electric field to polarize the filter fibers. Significantly, the Leiser publication does not recognize that charge accumulation on the filter fibers during operation will degrade performance over time. Further, the Leiser publication, like the Jaisinghani et al. patents, teaches coating a portion of the pleated filter media which results in a non-uniform distance between the ground electrode and the upstream ionizing electrode. Accordingly, the Leiser publication provides an incomplete solution to the arcing problem and no increase in efficiency or long-term performance. Moreover, the conductive coating applied to the downstream pleats blocks airflow through that portion of the filter media, reducing the effective area available for filtering particles. Because airflow is blocked at the pleats, the air flow dynamics are altered which can distort the pleat shape and further reduce effectiveness of the system.
The electrically enhanced air filtration industry continuously seeks improvements in manufacturability and cost. Although electrically enhanced air filters have proven to have superior performance, mechanical filtration alone has a significant initial cost advantage because of the simplicity of design and the relatively low cost of replacement filters. Many electrically enhanced air filter designs involve specially formed filter media that adds conductive layers, paints, or inks to the filter media to enable electric fields to be established across the media. Jaisinghani et al., for example, requires a conductive layer on the downstream filter surface while Leiser requires a conductive paint applied to the filter media to establish the polarizing electric field. The electrically enhanced air filtration systems described in U.S. Pat. No. 5,549,735 and U.S. Pat. No. 5,593,476 are notable exceptions in that they teach a system with field electrodes that are proximate to but not necessarily attached to the filter media. While proximate electrodes simplify the filter design, it has been found that proximate electrode designs allow the accumulation of charge in the filter media. The present invention addresses these limitations of prior systems by providing a filter design that has the benefits of a field electrode in contact with the filter media to solve the charge accumulation problem while at the same time providing the manufacturing and cost benefits associated with proximity field electrodes.
In view of the above, there remains a need for systems and methods for making and operating electrically enhanced air filters and air filtration systems with improved efficiency. More specifically, there is a need for air cleaning and filtration systems that counteract the effects of charge accumulation during operation so as to provide high cleaning efficiency throughout a long life and in certain configurations support a germicidal effect. There is also a need for a filter media suitable for electrically enhanced air filters that is cost-effective and efficient to manufacture.